GM Muscle Car Tach Voltage Requirements
I've got a '69 Chevelle SS396 with the factory tach. I'm planning on replacing the stock points with a PerTronix Ignitor electronic ignition module, along with the PerTronix Flame-Thrower coil. These modules require an ignition-switched 12-volt DC source. The Flame-Thrower is capable of handling 12 volts as well. My plan was to bypass the stock factory resistance wire and run a new wire from the ignition switch to power the coil and module. However, I don't know if I can safely connect the factory tach, since it normally doesn't see more than approximately 6 volts (except when cranking). I would really like to keep the tach original, but I don't want to burn it out, either. If it isn't possible to run it at 12 volts, can a resistor be added in the tach circuit. If so, what value would it need to be?
Edgar Gaudet
Halifax, Nova Scotia, Canada

2/5Here, the yellow ammeter reads 0, indicating no current flow in the ignition circuit, yet (even with a resistance wire present) all three voltmeters show more than 12 volts (the minor differences are only accuracy discrepancies between meters). This confirms the brown tach pulse wire that connects to coil (-) will see no less than 12 volts when the points are open.

Edgar, it turns out that most muscle car-era GM factory tachs (including the one in your Chevelle) actually run on full 12-volt power. These GM tachs are a three-wire design: The main power-up wire supplies full system voltage fed to it by the ignition-switched, fuse-protected, gauges circuit. Like any normal electrical circuit, there's also a conventional standard ground terminal or wire for the tach. The tach's third wire (usually brown in color) is the one that actually connects to the negative (-) side of the coil; it serves as the tach's trigger or sensing circuit. Internal tach circuitry translates the ignition system open/closed-circuit pulse-signal received through the brown wire into needle movement on the tach's face. This brown wire will actually receive extremely high voltage spikes in excess of 12 volts under running (not just cranking) conditions-even with a stock resistor wire.

Huh? How is that even possible? There are two separate issues you raise that need to be addressed: First we'll look at voltage drop through the resistance wire. M.A.D. Enterprises' electrical guru Mark Hamilton points out that with any conventional resistor, the amount of resistance remains constant (equal to the resistor's rating in Ohms), except for small changes caused by a change in temperature. The amount of current flow (amperes) is the same in all parts of a closed circuit, but-with resistance serving as an obstruction to current flow-the amount of voltage present will be reduced at the downstream side of any resistance.

3/5In the ignition circuit, electrical current flows from the ignition switch, through the resistor wire and the ignition coil primary winding to the breaker points in the distributor. Only when the breaker points are closed will the circuit be complete to ground. The factory tach is a three-wire design, receiving a full 12 volts from the gauges fuse. It has a conventional ground terminal plus a third "sense" wire connected to the coil (-) terminal that sees 12 volts or more with the points open and near-zero volts with the points closed.

This is shown by Ohm's Law, which can be used to calculate the amount of voltage drop resulting from a specific amount of current flowing through a specific amount of resistance. As used to calculate voltage drop, Ohm's Law is:

VOLTS = AMPS × OHMS

When the distributor points are mechanically open, the result is the same as with any open electrical switch: An open circuit condition exists because there is no longer a path to ground through the distributor; with no connection to ground, there is no longer any current flow through the circuit; with no current flow (0 amps) Ohm's Law tells us (because the product of multiplying anything by zero is zero) there cannot be any voltage drop in the circuit up to the location where the breaker points have the circuit open; with no voltage drop, you will still have full system voltage.

4/5Battery voltage (red meter) still exceeds 12 volts, but with 3.01 amps flowing through the fully closed circuit, there's only 5.55 volts at coil (+), indicating a voltage drop of 6.59 volts across the ballast resistor. With the coil consuming most of the remaining voltage, the left gray meter hooked to coil (-) reads near zero (0.64 volts). You'll always see significant voltage drop between coil (+) and (-) when the points or electronic ignition switch is closed.

0 AMPS ×2.19 OHM = 0 VOLT (drop)

It's only when the breaker points close (during the dwell period) that the circuit to ground through the distributor is completed and current flow causes voltage drop across the resistor wire.

With 12.14 volts coming from the ignition switch and a 6.59-volt drop occurring across the resistance wire, 5.55 volts would now be measured at the coil (+) terminal (12.14 - 6.59 = 5.55).

5/5The PerTronix Ignitor adds a new red wire to power its electronic switch inside the distributor. For optimum performance when running this ignition with PerTronix's recommended Flame-Thrower coil, remove the resistor wire or ballast resistor between the ignition switch and coil (+) and connect the new red wire directly to coil (+) (top). If running a stock coil, prevent it from overheating by splicing the new red wire into the full 12-volt, ignition-switched wire before the stock resistor (right).

However, the voltage at coil (-), the ground side of the circuit, will no longer be the same value as the (+) side. In fact, in the now complete circuit, it will be near zero because most of the volts are used up by the coil (and its internal resistance). Any slight remaining voltage at coil (-) will be due to the slight, but further downstream, resistance in the copper-conductor distributor lead wire and the resistance of the conductors in the breaker points.

These values correspond to some real-world measurements taken on a classic-era GM muscle car, as shown in the accompanying photos. In these photos, the red voltmeter connects to a full 12-volt source (the same junction that feeds power to the ignition switch); the yellow ammeter, in-series between the coil (-) and the distributor lead wire so it can measure the circuit's current-flow; the righthand gray voltmeter, to coil (+) where the resistor wire connects; and the left-hand gray voltmeter, to coil (-), where the wire to the distributor breaker points and (if present) the brown tach wire also connect. The full-page photo shows actual measurements with the points open; the smaller photo (page 114) shows the points-closed measurements.

The second point is the voltage seen at the tach. One thing these voltmeters won't show is when the points open and the coil's internal magnetic field collapses, pushing high voltage through the secondary circuit, there are also momentary, millisecond voltage spikes from 12 volts to as high as 250 volts at coil primary (-). Only an oscilloscope can detect them, and they're needed for correct tach function.

The bottom line is that there is no voltage drop at the resistance wire or the primary-side internal coil windings during the open-circuit portion of the ignition cycle. That means with or without an original resistance wire in place, the brown tach sense wire will be exposed to 12 volts minimum when either the points or electronic ignition switch are in an open-circuit condition. Only during the closed-circuit portion of the ignition cycle will there be a significant voltage drop at coil (-) due to internal coil resistance (again, whether or not a resistor wire is present).

So, no worries about tach not tolerating high voltage. But when deciding whether to discard the ballast resistor in a points-to-electronic ignition conversion, you do need to take into account whether the coil can operate safely under full system voltage without overheating. Standard PerTronix Ignitor ignitions will work with either the stock points-ignition coil or PerTronix's proprietary coil. In general, for optimum performance the coil's amount of internal resistance should ideally be closely matched to the overall ignition system characteristics as recommended by the system manufacturer.

Another point to remember is that if you are doing a full 12-volt electronic ignition conversion and testing for the presence of a resistance wire in the system, you won't get a valid voltage reading with the ignition key on if the points or electronic ignition switch happen to be in the open position, or if you are just checking for voltage with the ignition feed wire disconnected from the distributor. To get a correct reading, the breaker points must be in the closed position with the ignition in the "on" position. If there is no distributor in place, connect coil (-) to ground. A running engine will not serve for a voltage-drop test across a resistor wire. Cranking over the engine without starting it won't do it, either, because the yellow bypass wire from the starter solenoid R terminal delivers full system voltage to coil (+) during crank.

As for where to bypass, on most GM cars like yours, the portion of the ignition feed wire on the passenger side of the dash will be a nonresistor, 12-gauge, pink wire (as shown in the schematic); only the wire on the engine side of the firewall has resistance characteristics (typically, it'll be a 20-gauge purple or white wire).

Finally, PerTronix reports that a very small percentage of GM tachs may behave erratically with its system. This is not due to voltage, but because of the wave shape of the ignition system's pulse output. If that's the case, either of the following fixes installed in series between coil (-) and the tach should work: one 0.01 microfarad 1,000 VDC capacitor, or two 400 VDC 3A diodes (Radio Shack PN 276-1144 or equivalent). The band on the diodes should face the tach.

The preceding advice is valid only for a conventional inductive ignition circuit-be it points or electronic. It does not apply to a Capacitive Discharge (CD) system (including MSD multispark systems). They are completely different. Never connect factory (or even most aftermarket) electric tachs directly to the ignition coil terminals on a CD system. The capacitors in a CD box can deliver energy bursts to the coil's primary windings that exceed 400 volts. Obviously, that would completely fry the tach. CD system makers usually include a separate, dedicated tach terminal on their CD modules.

This advice also does not apply to certain early GM inductive two-wire tachs (typically found on some '67 or earlier models) , which won't work properly without the original system's designed-in ballast resistor present.

Finally, remember that it works is not the same thing as it's accurate. Old-school stock tachs are notoriously inaccurate, even in the original production configuration. If you are relying on the tach for critical shift-point information, an instrument restoration expert such as Redline Gauge Works can preserve the old tach's outward appearance but thoroughly update and make accurate its internal circuitry using modern electronic components (Redline uses VDO guts).

Just so everyone's on the same page, a cam's LSA or lobe-separation angle (aka the lobe-displacement angle or lobe spread) is the distance in camshaft degrees between the point of peak lift on the intake lobe and the peak lift on the exhaust lobe. On a conventional single-cam V8, LSA is ground into the cam when it's manufactured and can't be changed by the end user. The reason for the difference in LSA is related to the inlet runner length, and, to a lesser extent, the use of fuel injection.

Looking at runner length first, for the purposes of this discussion we mean the entire inlet runner length from the intake manifold's plenum floor to the intake valve seat in the cylinder head. Short runners tend to work better with tighter lobe-separation angles; longer runners like wider separation. A runner that is longer and/or has a longer length compared to the port's cross-sectional area (has a higher ratio) has more inherent inertia and can tune better at low rpm without the need for the increased overlap provided by a similar cam with a tighter lobe-separation angle. An extreme example is a Harley-Davidson motorcycle engine: It has almost no runner length, and responds well to 102- to 104-degree LSAs. Modern, LS-style small-block cams typically have 112-degree-or-wider LSAs because their combined inlet port and intake manifold runner lengths are longer than the classic Chevy small-block, which usually likes 110-degree-or-narrower LSAs in normally aspirated applications.

Bolting on a different inlet manifold design that significantly changes the overall runner length means the cam LSA should also change to achieve optimum performance. For example, a serious 5.0L Ford running a single-plane intake manifold runs strong on a 104- to 106-degree LSA, but add a typical long-runner EFI inlet to the brew and a similar performance-level 5.0L wants a 110- to 116-degree LSA.

Their generally longer runner length is not the only reason fuel-injection apps work well with wider LSA cams. A carburetor meters fuel under the principles of vacuum differential and needs a strong signal to get fuel moving through the carb's main jets. Tightening up the LSA provides the needed stronger signal. On the other hand, an EFI system meters fuel in response to a preprogrammed computer, so a strong initial signal isn't required. A wider LSA also tends to promote a more stable idle, which makes the EFI system's electronic sensors happier and (with less overlap) tends to lower emissions.

Epoxying Runners, Mopar Six-Pack EFI
I am building a '41 Chevy four-door street rod. It will have a Dodge (yes, Mopar in a Chevy) 360 with 340X heads and Six-Pack intake (will bore 0.030 over and stroke to 4 inches to build a 408), backed by a 46RH transmission. I picked up the Six-Pack manifold used after it had already been ported for W-2 heads. The manifold ports are bigger than the already-ported 340 heads. Is there an epoxy or liquid aluminum that can be safely used to build up the manifold ports so they can be ported to match the 340 heads?

Second question: I would like to put fuel injection on the manifold that would work in stages just like the original Six-Pack. Any advice or sources you can recommend? Don't tell me to stick with a four-barrel, because it's not just about cost or performance. Funds are not unlimited, but nostalgia, the look, and being different are important.
Jim Graham
Chesapeake, VA

Hard-core racers have used various epoxies for years, but the conventional wisdom on epoxying ports and runners was that it just wasn't durable enough for long-term street use. Heating and cooling cycles eventually cause a loss of adhesion, resulting in engine ingestion and major parts damage. But Joe Mondello at the Mondello Tech Center says that epoxy compounds have come a long way. "We now have a stable, two-part epoxy called A788 that maintains good adhesion if properly prepped." It works on intake head and manifold ports (but not on exhaust ports). It's not affected by gasoline, but it should not be used with methanol. To apply it, the surface must be totally clean and free of oil. Prep the walls with coarse-grit sandpaper. After taking a set, it retains a hard finish but ports and polishes with ease. Call the Tech Center and ask for Joe; he'll talk you through the critical prep process step by step. A788 is available in an 8-ounce kit as well as in 1-quart cans.

That said, brand-new Six-Pack intake manifolds for big- and small-block Mopars are available from Mopar Performance, Chrysler's performance parts division (order PN P4529054 for your 360). Sell your existing modified intake to someone running W-2 heads and you may even break even on the deal.

As for EFI, it's your lucky day, Jim. F&B Performance sells Six-Pack EFI setups for small- and big-block Chryslers. These kits include three two-barrel billet throttle-bodies of F&B's own design that bolt to the original two-barrel Holley carb mounting pattern, fuel rails, injectors, progressive linkage, and a choice of air cleaners. F&B can even supply the intake.

Speedo Restoration
I have a '69 427 Vette convertible. It has the speed-warning speedo. Everything works OK except for the speed warning. There is a plastic socket where the reset cable goes into (inside the speedo). Somewhere in its time it was broken, and now the speed warning cannot be reset. I hope you can help-you always have an answer for the most obscure items.
Cef Saiz
Nanuet, NY

Cef, it's only obscure until it's your car or part that's down. You'll need to contact an instrument and speedometer restoration specialist to get the unit repaired. There are a number of them around the country. One shop that's about a three-hour drive from your location is Instrument Specialties in Rhode Island, which does both mail-in and drive-in repairs on most gauges, instruments, and speedometers from the early 1900s through the early 1980s (including your Corvette). It has relocated, and the contact info on the website hadn't yet been updated when this was written. The corrected info is shown below.

LS Management On LT1
Is there a kit for installing an LS1 engine management system onto a Gen II LT1?
Erin Wickizer
Shawnee, OK

EFI Connection specializes in LS fuel management systems for Chevy small-block Gen I (classic), LT1/LT4 Gen II engines, and late 7.4L big-blocks (not Mark IV or Gen V). It has all the parts you need to run the modern LS computer, sensors, and coil ignition system. Yay-no more trouble-prone LT1 Opti-Spark distributor!